Foam coating of press fabrics to achieve a controlled void volume

ABSTRACT

This invention is directed to coating press fabrics to achieve a controlled void volume. More particularly, this invention is directed to a method of modifying a press fabric for a papermaking machine which comprises the steps of: 
     (a) applying a thin layer of a polymeric foam to the surface of a press fabric; 
     (b) drying said foam to form a coated press fabric; and 
     (c) curing said coated press fabric.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of pending U.S. patentapplication Ser. No. 566,829, filed Aug. 14, 1990, now abandoned, whichin turn is a division of U.S. patent application Ser. No. 265,258, filedOct. 31, 1988 now U.S. Pat. No. 5,118,557.

FIELD OF THE INVENTION

This invention is directed to press fabrics having a foam coating. Morespecifically, this invention is directed to the coating of press fabricsto achieve a controlled void volume and permeability.

BACKGROUND OF THE INVENTION

Papermakers' press fabrics are endless belts of fibrous material usedfor conveying a wet paper web, delivered by a wet-type papermakingmachine, from a forming zone, through a pressing zone, to a drying zone.At the pressing zone there is usually provided rotating cylindricalsqueeze rolls between which the freshly formed paper web is passed. Asthe web enters the nip of the rolls, water is squeezed from the paperand is accepted by the press fabric upon which the paper is conveyedthrough the nip.

Papermakers' press fabrics are well known. Such fabrics are typicallyformed from materials such as wool, nylon, and/or other syntheticpolymeric materials and the like. With such fabrics, the paper web,after passing through the nip of the pressing rolls, usually stillcontains an appreciable amount of water, which adds substantially tomanufacturing costs due to the high energy required to evaporate thewater during the subsequent drying stage. Increasing and/or maintainingfor a longer period of time the permeability and water removalcapability of the press fabrics would thus be highly advantageous inthat manufacturing costs would be reduced. Other objectives includesmoother surface, free of needle tracks; increased sheet contact area;and uniformity of pressure distribution.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an improved press fabric.

It is also an object of the invention to provide a method of treating apress fabric to achieve a predetermined permeability.

It is a further object of the invention to provide a relatively easy andpredictable method of adjusting the void volume of a press fabric.

These and other objects of the invention will become more apparent inthe discussion below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a graph of sheet solids content versus fabric wrapcaused by various press fabrics, including an embodiment of theinvention; and

FIG. 2 represents a graphic depiction of the relationship between papersheet smoothness and press load for various press fabrics, including anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a method ofmodifying a papermaker's press fabric to adjust its permeability. Morespecifically, a papermaking press fabric is treated with one or morelayers of polymeric foam that are dried and then cured.

The press fabrics to be modified include those press fabrics known inthe art. Typical such fabrics are described in, for example, U.S. Pat.Nos. 2,354,435, 2,567,097, 3,059,312, 3,158,984, 3,425,392, 3,617,442,3,657,068, and 4,382,987, and British Patent No. 980,288, all of whichare incorporated herein by reference. Preferably batt-on-base orbatt-on-mesh press fabrics are coated according to the invention.Examples of these include a batt-on-base press fabric known as DURASORB®and batt-on-mesh press fabrics known as DURAMESH®, DURACOMB®, orDURAVENT®, all of which are available from Albany InternationalCorporation.

It will be apparent to those skilled in the art that coating of othersubstrates in the manner described would result in structures havingsufficient paper sheet dewatering capabilities. Examples of some ofthese substrates include: woven and non-woven structures, with orwithout needled fiber; composite structures consisting of severalfibrous configurations; air-layed and wet layer fibrous sheets; and thelike.

Useful resin compositions include synthetic, flexible, polymeric resinfoams. Useful are foams based upon polyurethanes, polyether, polyester,polysilicone, polyacrylic, polyvinyl chloride, polyisocyanate, epoxy,polyolefins, or polyacylonitrite rubber foam, and the like. Also, acombination of two or more such elastomeric resins can be utilized.Typical of useful resin compositions are Emulsion 26172 (an acrylicemulsion representative of a large series of emulsions available from B.F. Goodrich) and Permuthane HD2004 (a water-based polyurethane emulsionavailable from C. L. Hauthaway).

It is recognized that the resin composition can be solvent; water-based;high solids (that is, containing little or no solvent); or a combinationof solvents or co-solvents that results in complete or partialsolubilization and/or suspension of the resin particles. This would alsoinclude plastisols, water-based, and other emulsions.

In addition, the foam can contain one or more surfactants, emulsifiers,stabilizers, or the like. Examples of such additives include ammoniumstearate, ACRYOL TT678 (an acrylic polymeric compound used as athickening agent, available from Rohm & Haas), ASE 60 (an acrylicpolymeric compound used as a thickening agent, available from Rohm &Haas), TAMOL (an organic salt, dispersant, used to stabilize the mixtureprior to and during foaming, available from Rohm & Haas), TRITON (anonionic detergent used herein as a foaming agent, available from Rohm &Haas), PLURONIC L62 (a nonionic detergent, used herein as a foamingagent, available from BASF), and the like.

The foam structure in the final form, can be either an open (i.e.,reticulated) or closed cell structure, or a combination thereof. In somecases collapse of the foam during curing results in a coating orbridging of the substrate fibers. Any of these forms or combinationsthereof result in alteration of the substrate characteristics.

According to the invention a foam is applied to a surface, or surfaces,of a press fabric, is allowed to dry, and is then cured. The drying andcuring could be performed in separate steps or simultaneously. In somecases, it may be desirable to calender the fabric after drying andbefore the curing step.

The foam could be applied by any number of known procedures, whichinclude, for example, blade coating techniques which can be on roll, offroll, or table; squeeze coating; transfer coating; spraying; kiss orapplicator roll; slot applicator; and brush application. A single layercan be applied or multiple layers of the same or different foamformulations can be applied to obtain a given final result. In apreferred embodiment of the invention the foam is applied in a series ofvery thin layers with minimal overlap. For example, the foam could beapplied in from about 2 to 10 layers, each of which is from about 1 to10 mm thick, with an overlap of from about 1 to 80 cm, preferably fromabout 3 to 50 cm. Preferably the foam is applied to the press fabric asa thin continuous layer.

The resultant foam may reside entirely upon the press fabric to theextent of 90% or more extending above the surface fiber plane, or it maybe partially embedded into the surface to the extent of about 50%,leaving 50% above the surface. In the alternative, the foam may beprimarily embedded in the press fabric, penetrating partially or whollyinto the press fabric.

Each layer is dried. After the topmost layer is dried, the coated pressfabric is cured, for example, by air drying at room temperature for asufficient length of time or at elevated temperatures for from about 1minute to 5 hours. The temperature and time for drying or curing will bedependent upon the foam employed, manufacturing conditions, and thelike.

The following examples are intended to illustrate the invention andshould not be construed as limiting the invention thereto.

EXAMPLES Example 1

A water-based polyurethane emulsion having 40% urethane solids emulsionwas prepared, and the emulsion was then foamed to a 6 to 1 blow ratio.The resultant foam was used to coat a DURAVENT® double layer,batt-on-mesh press fabric (available from Albany International Corp.)with repeated passes.

By use of a Frazer air permeability tester, the air permeability wastested. The results are set forth in the following table:

                  TABLE I                                                         ______________________________________                                                       Thickness  Air Permeability                                           No. of  of Applied cfm/sq. ft.                                         Sample Coats   Layers     After Drying                                                                           After Curing                               ______________________________________                                         A*    0       --         (50.5)   --                                         B      l       25 mils    30       32                                         C      2       25 mils    15       14                                         D      3       15 mils    10       10                                         ______________________________________                                         *Control                                                                 

Note the permeability was uneffected by the curing step. It is possibleto continue adding foam layers until the desired permeability isobtained.

Example 2

Foamed water-based urethanes have been considered as a replacement for100% solids polyurethane for many reasons, for example, control ofoverlap when coating endless structures or when better predictability ofvoid volume is required. As is reflected below, overlap can becontrolled rather closely. Those familiar with the art will recognizethat "100% solids polyurethanes" are those containing little or nosolvent and are referred to as "high solids" or "100% solids"polyurethanes.

In the coating of a press fabric with foam in multiple passes, it wasfound that for the particular foam used, data fit the empiricalequation:

    ln (air Perm)=ln (original Press Fabric Air Perm)-(AP+BP.sup.2)

where A and B are constants (but not the same for all materials) and Pequals the number of coating passes. This formula gives an indication ofthe extent to which overlapping coatings changes the permeability. Afterseveral coatings the small change due to overlap would not be expectedto affect sheet properties.

A coated press fabric was prepared by applying layers of a water-basedpolyurethane foam to a DURAVENT press fabric. The measured airpermeability measured and calculated data are set forth in the followingtable:

                  TABLE II                                                        ______________________________________                                                      Air Permeability (cfm/sq. ft)                                   Sample No. of Layers                                                                              Observed   Calculated                                     ______________________________________                                         A*    0            92         --                                             B      1            77         76                                             C      4            38         38                                             D      6            24         22                                             E      8            10         11                                             F      10            4          5                                             ______________________________________                                         *Control                                                                 

Example 3

Laboratory trials were made using polyurethane foam made from awater-based emulsion from Permuthane, said foam being applied toDURACOMB® double layer, batt-on-mesh press fabric, Style 5710 fabric,and DURAVENT press fabric (all available from Albany InternationalCorp.). A relatively low blow ratio foam (2.7 blow ratio) was used, andseveral layers were applied. Air permeabilities were measured after eachpass. Each fabric sample was run in duplicate, and the data from bothruns are set forth below in the following table:

                  TABLE III                                                       ______________________________________                                                           Air Permeability                                                              (cfm/sq. ft.)                                              Sample Fabric       Uncoated 1 Coat                                                                              2 Coats                                                                             3 Coats                              ______________________________________                                         A*    DURACOMB     125      --    --    --                                   B      DURACOMB     --       102   92    66                                   C      DURACOMB     --       116   98    78                                    D*    5710         427      --    --    --                                   E      5710         --       309   47    18                                   F      5710         --       302   48    13                                    G*    DURAVENT      21      --    --    --                                   H      DURAVENT     --        20   16    10                                   I      DURAVENT     --        20   18    13                                   ______________________________________                                         *Control                                                                 

The data indicate that the reproducability is good. It is interesting tonote that the open structure Style 5710 fabric was closed up more witheach pass than the DURACOMB fabric, indicating specific formulations foreach type of fabric to be coated are necessary.

Example 4

Two sets of fabric samples, SCREEN TEX (available from AlbanyInternational Corp.) and Style 5710, were coated with a foam made fromB. F. Goodrich acrylic latex. The objective was to make a series ofsamples with air permeabilities of approximately 40, 60, and 80cfm/sq.ft. The results are set forth in the following table:

                  TABLE IV                                                        ______________________________________                                                                       Air Permeability                               Sample  Fabric      No. of Layers                                                                            (cfm/sq. ft.)                                  ______________________________________                                         A*     SCREEN TEX  0          405                                            B       SCREEN TEX  2          87                                             C       SCREEN TEX  4          55                                             D       SCREEN TEX  6          42                                              E*     5710        0          478                                            F       5710        2          80                                             G       5710        4          70                                             H       5710        6          40                                             ______________________________________                                         *Control                                                                 

Those skilled in the art of press fabric making will recognize that thetarget values were closely obtained for each series.

Example 5

Trials were run on a pilot paper machine of a series of press fabrics todetermine the effect on sheet dewatering and sheet printabilitycharacteristics of newsprint. Typical newsprint furnish was used. Thepress arrangement was three separate presses, each clothed with its ownpress fabric, commonly referred to as a "Twinver Press". Four pressfabrics were submitted and classified as coarse, medium, super smooth,and coated.

The coated press fabric embodied the medium fabric substructure and battfiber, but with a urethane emulsion foam coating. The purpose was toexamine whether the coating would allow coarser structure, especiallycoarser batt fibers, to be used in press fabrics, with no loss inproperties. It was hoped that some improvements would be observed.

The data obtained are shown in FIGS. 1 and 2. FIG. 1 represents the datataken on newsprint solids content after the last press, using slightlydifferent fabric run take off angle geometry. This increase or decreaseof contact time between press fabric and paper sheet determines thedegree of "rewet" or the amount of water once mechanically removed, thatis, removed from the paper sheet by the fabric, that is reabsorbed bythe paper sheet at the fabric/sheet interface.

As can be seen, under the normal running conditions the medium pressfabric produced the highest sheet solids content. The X is the conditionmeasured for the foam coated fabric. It was not measured under allfabric run configurations. As can be seen, the solids were as high aswith any press fabric tested.

As shown in FIG. 2, a ranking of "0" (zero) is that sheet surfacesmoothness that would be obtained by pressing the paper sheet against asmooth granite press roll. It is the objective to supply textilestructures that will adhere close for this "0" (zero) ranking underoperating conditions.

As can be seen in FIG. 1, no negative effects were observed on sheetdewatering. A considerable improvement in sheet smoothness for thecoated fabric was noted versus the medium fabric, and the coated fabricproduced nearly as smooth a sheet surface as did the supersmooth fabric,according to the data in FIG. 2.

It should be noted that the supersmooth fabric, which incorporated avery fine base fabric, and fine batt (all 3 denier fiber), would causeconsiderable operating problems on a production paper machine due tofilling, compaction, and wearing away of the 3 denier surface fiber.Sheet following wherein the sheet does not release cleanly from thefabric after the press nip would also be expected. None of thesetendencies was observed with the coated fabric during the evaluation.

Further laboratory data derived from three trials confirm that on apressure sensitive furnish such as newsprint, smoothness increasesattributable to the fabrics are a result of increased surface contact atthe interface between the paper sheet and the press fabric. It thereforefollows that the improved sheet smoothness values obtained were due tothe increased contact area of the foamed press fabric versus a fabricwith a normal textile fiber surface.

Hand sheet studies have long confirmed that porous, uniform surfaceswith a high percent contact area show greater paper sheet water removalby mechanical action under conditions of pressure controlled pressing.Many studies on dewatering published in the literature confirm this.Whether the effect is due to reducing rewet in the nip or post nip or tohigher sheet dewatering in the nip is still being argued by therespective schools of thought. Regardless of which mechanism prevails,the porous foamed surface pressing media disclosed herein with itshigher surface contact area, its controlled porosity, and void volumewill fit either theory.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, however, that other expedientsknown to those skilled in the art or disclosed herein, may be employedwithout departing from the spirit of the invention or the scope of theappended claims.

I claim:
 1. The method of modifying a press fabric for a papermakingmachine to improve water removal characteristics, to impart a betterfinish to the paper, and to enhance paper making characteristics, whichmethod comprises the steps of:(a) applying a thin continuous layer of apolymeric foam to the upper surface of a press fabric; (b) drying saidfoam; (c) repeating steps (a) and (b) one or more times sufficient toform an effective coating on said press fabric; and (d) curing the thuscoated press fabric.
 2. The method of claim 1, wherein the polymericfoam is primarily a polyurethane foam.
 3. The method of claim 1, whereinthe polymeric foam is primarily a polyacrylic foam.
 4. The method ofclaim 1, wherein the foam is air dried at room temperature.
 5. Themethod of claim 1, wherein the modified fabric is cured by air drying atroom temperature.
 6. The method of claim 1, wherein the modified fabricis dried by heating at elevated temperature for from about 1 minute to5hours.
 7. The method of claim 1, wherein the modified fabric is cured byheating at elevated temperature for from about 1 minute to 5 hours. 8.The method of claim 1, wherein the polymeric foam comprises one or moreresinous materials selected from the group consisting of polyurethanes,polyacrylates, polyethers, polyesters, polysilicones, polyvinylchlorides, polyisocyanates, and polyacrylonitrile rubbers.
 9. The methodof claim 1 wherein the polymeric foam is an open cell foam.